1) Field of the Invention
Embodiments of this invention relates generally to a structure and method for forming a semiconductor memory device and more particularly to a SONOS memory device.
2) Description of the Prior Art
Semiconductor memory devices in general can be divided into two types: a random access memory (RAM) and a read only memory (ROM). The RAM, such as a dynamic random access memory (DRAM) and a static access memory (SRAM), is fast to input and output data and is volatile such that previously stored data is removed after a predetermined time has passed or power turn-off. Although the ROM keeps data once it is stored, it is slow to input and output data. The ROM includes a Rom, a programmable ROM (PROM), an erasable PROM (EPROM), and an electrically erasable PROM (EEPROM). Recently, there is an increasing demand for EEPROM being capable of electrically programming or removing data. The EEPROM or a cell of a flash memory having a bulk erasing function has a stack type gate structure in which a floating gate and a control gate are stacked.
Meanwhile, a quantum trap device has been the center of public attention as a nonvolatile SRAM or a flash memory for a next generation device. The quantum trap device has a silicon-oxide-nitride-oxide-silicon (SONOS) structure. Operation of the SONOS device having the SONOS structure is divided into reading, programming, and erasing. In the case of programming, when a program voltage is applied to the gate of the transistor and a drain of the cell, a hot electron is formed and then surrounded by a nitride film being at an area adjacent to the drain. Thus, a threshold voltage of the transistor is increased. Therefore, data programming is performed.
In case of erasing operation, when the gate, the drain and the source are opened and an erasing voltage is applied to the semiconductor substrate, the electrode surrounded by the nitride film is pushed away from the semiconductor substrate. Thus, the threshold voltage is lowered. Therefore, the data erasing is performed.
In the case of reading operation, a reading voltage is applied to the gate, and an electric current flowing through a space between the source and the drain is sensed by a sensing circuit. Accordingly, the data reading is performed.
In the case of the SONOS, since program/removal characteristics and retention characteristics must be traded off with each other, it is impossible to improve the program/removal characteristic and the retention characteristic simultaneously. Accordingly, a proper trade off point between the removing characteristic and the retention characteristic is desired so as to optimize an oxide-nitride-oxide layer of the SONOS.
The importance of overcoming the various deficiencies noted above is evidenced by the extensive technological development directed to the subject, as documented by the relevant patent and technical literature. The closest and apparently more relevant technical developments in the patent literature can be gleaned by considering the following.
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